RexS

I have a 90 Coupe -6 sp manual L98 with 101K miles on it that had sat unused for 5+ years just before I bought it. A mechanic had replaced the battery, fuel pump and filter and the car was running.

I bought it and gingerly drove it home to check fluids and "the usual stuff". A couple of days later I took it out for spin and noted that it started to stumble after a mile or two. I'd give it throttle and it would bog down so much so that I could not get through a traffic light intersection. It would idle fine. I idled it home and checked for codes...nothing but 12s (light comes on initially at start, so I know it is not burned out).

I decided (rather unscientifically) to finish the "replacement" of the fuel system and ordered new injectors, the guts for the fuel pressure regulator, a new EGR valve and air intake temp sensor since they are all under the intake plenum. I also replaced the MAP sensor and the thermostat (195). Starts nice but still has the stumbling problem.

So I bought a fancy mechanical fuel pressure gage and taped it to the windshield. Very informative. Insert key and turn to "run" position and the fuel pump drives the pressure up to mid 40# pressure but it immediately begins to drop. If I turn key to "start" position, the car fires and the fuel pressure rises to a steady 38#. I drive down the block and it stays around 38# but varies a bit. When stone cold, the car can go through the first 3 or 4 gears without problems. When I get to the boulevard and possible 50mph, as I go through the gears the pressure drops...20# - 15# and the engine bogs down. Clearly, as the fuel pressure drops, the performance of the engine drops.

So I pulled out the fuel pump this afternoon and it looks good. No blown or leaky lines that I could see. It appears to be an Airtex E3265 with a production date in mid 2010. That appears to be the right fuel pump for this application according to the Airtex application charts.

The only part I did not observe personally is the fuel filter. I know the PO was billed for the work (I have the invoice) and I believe it to be new.

My question: Should fuel pressure rise to the preset of the fuel pressure regulator and stay pegged there (with little fluctuation) or can it vary a little bit? What could influence the fuel pressure in this way?

DanielRicany

Pull the vacuum hose off the regulator, if there is has in it, the diaphragm in the regulator has broken and you need to replace the regulator. If there is no gas then I would check your fuel pump wiring as it might be an intermittent problem with excessive resistance in the wires.

hooked073

did you replace the fuel filter??????

hooked073

sorry I missed where you said you did.

hooked073

I wonder if he drained the fuel tank?? if not I would not be opposed to draining the tank and replacing the fuel filter. there are few things that can cause what you are saying and filter bad pump or regulator cover most of them... If nothing else replace the filter and see what you get.

hooked073

you will get some verations but no where near what you are saying.

RexS

the regulator has a fresh diaphragm in it. The old one was ratty but functioning and I rebuilt it while installing the injectors.

I looked at the gas in the tank when I pulled the fuel pump and it did not seem unduly cruddy. The sock on the fuel pump was quite clean.

The fuel filter.... it is reportedly new. I believe it is.

RexS

I should note that the stumbling occurred before and after the handful of parts that I dropped in to her...including the regulator rebuild.

DanielRicany

Inspect your fuel lines for bends or dents. Take a look at the top of the tank unit and make sure that all the hoses are clamped on securely. The one at 12 o'clock should have two clamps on it.

Giamanut

you say it appears that the stumbling is a result of FP but note as you go to higher gears load increases and more fuel is required, the ecm fires the injectors way more often when load increases. This require the fuel pump to get the fuel there faster. Increased velocity = decreased pressure. Pressure will drop as load increases. That is not to say you do not have a fp problem It does sound lean but the fact that you have pressure while under load does indicate IMHO that your pump is keeping up with the injector demand. I think you have bank fire injection. I would check Injector wiring.

95BLKVette

Think you have a leak in either the ball check valve in the fuel pump or in the pulse damper assembly that is mounted between the fuel pump and steel supply fuel piping. My damper went bad and had your same issue with lack of performance as the fuel pressure never gets to the pressure regulator!

I checked mine by pulling the in tank level/pump assembly out and placing in a plastic tub with about an inch of fuel in it. Clamped a small piece of fuel hose to the outlet pipe and pinched it off with small vice grips. Put the tub on the back of the car and hooked up electrical connection. Turned on key and the pump pressurized and the damper squirted a stream of fuel out. Mine was leaking between the steel pipe and rubber bladder.

I removed the damper and replaced it with a piece of fuel tubing with two hose clamps.....no more fuel pressure problems.....holds pressure at rail rock steady at key on

Hope this helps.....btw, this is in a troubleshooting chart in the service manual

RexS

I'm viewing pressure with an analog gauge connected directly to the rail so I doubt that I have a bank dropping out under load. I can see the pressure drop as the banks demand more fuel and the pressure ain't there.

The fuel pump has the all the right clamps (doubled at 12 o'clock) and is missing the check valve and the dampener. I understand that is common on rebuilds. The lack of a check valve would account for the initial drop in pressure after the key is in run position but would not effect running once the engine has fired and the dampener is a sound dampener (damper?). The short section of hose that replaces these items is new and in excellent condition and properly clamped.

Does anybody know how much the fuel pressure regulator can vary the fuel pressure based on vacuum? Does high vacuum increase fuel pressure?

DanielRicany

You Should See A Difference Of 3-4 Psi But It Should Average Around The proper Psi. And No, Vacuum Keeps It Lower, As You Release Vacuum By Opening The Throttle Plates, The Pressure Increases.

Giamanut

I believe RexS is correct. A little to much FP drop to be injector related I was actually thinking about that backwards. Anyways let us know what the fix is, good luck.

95BLKVette

With vacuum line off, fuel pressure should stay pegged at 45 psi.

Something is leaking as you have bleed down when you first turn key on....it should hold at 45 after pump shuts off.

Try this, clamp off return line at fuel tank and turn key on. If pressure goes up to the 40 mid range but holds there then your regulator is leaking internally or one of your injectors....easy to find out which one by pulling injectors

RexS

Son of a gun...

Bad new fuel pump!

I ordered a Delco OEM and it was a nice direct fit w/o any redneck engineering. I took a spin with the ol' fuel pressure gage taped to the windshield and it never missed a beat. Opening FP was 38# and under load it climbed into the low - mid 40s.

Great! Now I can move on the the ABS...Radio...seats.....